The present invention relates to a cruise control apparatus for a vehicle capable of making the vehicle cruise at a constant speed.
In general, a cruise control apparatus for a vehicle is constructed such that a control unit drives, upon receipt of a command signal which is generated by a cruise control switch when it is operated by the driver, a throttle valve in an intake pipe of an internal combustion engine to open and close through the action of an actuator so as to make the vehicle travel at a target speed.
An example of such a cruise control apparatus including a cruise control switch and a control unit is diagrammatically illustrated in FIG. 3. The known cruise control apparatus illustrated includes a cruise control switch 1 and a control unit 2. The cruise control switch 1 is, together with a horn switch 3, incorporated in a steering wheel (not shown) and constitutes a steering wheel built-in switch circuit 5. The horn switch 3 is connected at one end thereof through a slip ring 6 to a horn relay 7 which is in turn connected to a power supply V.sub.1 and a horn 8. The horn switch 3 is also connected at the other end thereof to ground through a bearing in a steering shaft (not shown). The cruise control switch 1 includes a cancel switch 9 for cancelling a cruise control operation, a set/coast switch 10 for setting a cruise control operation during a non-cruise control operation as well as automatically decelerating the vehicle during a cruise control operation, and a resume/acceleration switch 11 for automatically resuming the vehicle to cruise during a non-cruise control operation as well as accelerating the vehicle during a cruise control operation. The set/coast switch 10 is connected in series with a resistor R.sub.1 to form a first serial connection. The resume/acceleration switch 11 is connected in series with a resistor R.sub.2 to form a second serial connection. The first serial connection including the set/coast switch 10 and the resistor R.sub.1, the second serial connection including the resume/acceleration switch 11 and the resistor R.sub.2, and the cancellation switch 9 are connected in parallel with each other. The cruise control switch 1 is connected at one end thereof to the control unit 2 through a slip ring 12, and at the other end thereof to a primary power supply V.sub.2 through a slip ring 4. Inside the control unit 2, an output of the cruise control switch 1 which is input to the control unit 2 via the slip ring 12 is connected through a resistor R.sub.3 to ground and at the same time is input to the respective first or positive terminals of comparators 13 through 15. The second or negative terminals of the comparators 13 through 15 are connected to the primary power supply V.sub.2 through corresponding resistors R.sub.4 through R.sub.6, respectively, and at the same time to ground via resistor R.sub.7. The voltages V.sub.H, V.sub.M and V.sub.L at the negative input terminals of the comparators 13 through 15, respectively, are predetermined so as to satisfy the following relationship: V.sub.H &gt;V.sub.M &gt;V.sub.L. A secondary power supply V.sub.3 for exclusively supplying power to the cruise control unit 2 is connected to the comparators 13 through 15. The output terminals of the comparators 13 through 15 are connected to a power supply of 5 volts via resistors R.sub.8 through R.sub.10, respectively, and the outputs V.sub.a through V.sub.c of these comparators are input to a central processing unit (CPU) 16.
In operation, when the set/coast switch 10 is turned on with the vehicle travelling under non-cruise control, the input voltage V.sub.i fed from the cruise control switch 1 to the control unit 2 is expressed by the following equation; EQU V.sub.i ={R.sub.3 /(R.sub.1 +R.sub.3)}.times.V.sub.2.
In this case, the input voltage V.sub.i is between V.sub.M and V.sub.L, so the output voltage V.sub.c of the comparator 15 becomes high while the output voltages V.sub.a, V.sub.b of the comparators 13, 14 both remain low. As a result, the CPU 16 operates to recognize a command signal from the set/coast switch 10, and accordingly drive the unillustrated throttle valve. Similarly, when the resume/acceleration switch 11 is turned on, the input voltage V.sub.i is expressed by the following equation; EQU V.sub.i ={R.sub.3 /(R.sub.2 +R.sub.3)}.times.V.sub.2.
In this case, the input voltage V.sub.i is between V.sub.H and V.sub.M, so the output voltages V.sub.b, V.sub.c of the comparators 14, 15 both become high while the output voltage V.sub.a of the comparator 13 remains low. As a result, the CPU 16 recognizes a command signal from the resume/acceleration switch 11, and accordingly drive the throttle valve. Further, when the cancellation switch 9 is turned on, the input voltage V.sub.i becomes substantially equal to the output voltage V.sub.2 of the primary power supply which is greater than V.sub.H. Thus, in this case, the output voltages V.sub.a, V.sub.b and V.sub.c of the comparators 13 through 15 all become low, whereby the CPU 16 recognizes a command signal from the cancellation switch 9, and drives the throttle valve accordingly. These operational relationships are tabulated in Table I below.
TABLE I ______________________________________ Output Voltages Control SW Input Voltages V.sub.i V.sub.a V.sub.b V.sub.c ______________________________________ Set/Coast SW {R.sub.3 /(R.sub.1 + R.sub.3)} .times. V.sub.2 0 0 5 Resume/Accel SW {R.sub.3 /(R.sub.2 + R.sub.3)} .times. V.sub.2 0 5 5 Cancellation SW V.sub.2 5 5 5 All SWs Off 0 0 0 0 ______________________________________
With the above-mentioned known cruise control apparatus, however, if the primary power supply V.sub.2 fails or wiring between the primary power supply V.sub.2 and the control unit 2 is cut or broken, the voltages V.sub.H, V.sub.M and V.sub.L at the negative input terminals of the comparators 13 through 15, respectively, all become zero and the voltage V.sub.i at the positive input terminals of the comparators 13 through 15 also becomes zero, so that the output voltages V.sub.a through V.sub.c of the comparators 13 through 15 becomes floating or indefinite, causing an abnormal situation that cruise control may be automatically set without the cruise control switch 1 being set by the driver. Similarly, if a soldered connection at a between the control unit 2 and the primary power supply V.sub.2 is broken or becomes a bad connection, the voltages V.sub.H, V.sub.M and V.sub.L all become zero, causing another abnormal situation that the operation of the control unit 2 becomes indefinite as in the case of the cruise control switch 1 being in its "OFF" range.